It is required to sign up for a playstation network account for PS3. The steps for the same are –
Go to the “Playstation Network” menu -> Sign up for Playstation Network ->Create a new account ->Continue -> Select your country, language and date of birth. For users of age 18+, a Master Account is created, else a Sub-account is created. Accept the terms of service and agreement policy. Enter a valid email address (log-in ID), password, and security question. Choose an Online ID(your username, as it would appear to all)
After signing up, the Friends menu will show options like Message Box.